Spray Droplet Size Distribution Research Articles

INFLUENCE OF FUEL PROPERTIES ON THE DIESEL INJECTION PROCESS IN NONVAPORIZING CONDITIONS

This paper presents an experimental investigation of the influence of fuel density, viscosity, and surface tension on the diesel injection process (spray development and droplet size distribution) in non-vaporizing conditions. The spray development was captured in a pressurized vessel with a high-speed camera in order to obtain the penetration length and the spray angle. The droplet size distributions were measured by laser diffraction. A fuel matrix of eight fuels was designed including independent variations of fuel density, viscosity, and surface tension. Experimental conditions, such as injection pressure and air density, were varied according to current diesel engine conditions. The analysis focuses mainly on the quasi-stationary phase of the injection event. Results indicate that viscosity and to a lesser extent fuel density affect the spray characteristics. Increasing fuel density and viscosity by, respectively, 100 kg/m3 and 6.5 mm2/s induces longer spray tip penetration (up to 7%) with a narrower spray angle (drop of up to 3 deg) in quasi-stationary conditions. An increase in the Sauter mean diameter (up to 23% or 3 µm) was also observed. An increase in surface tension from 18 to 30 mN/m did not induce any significant modifications in the droplet size or in the spray development. Correlations, some of which are based on existing ones, are proposed to estimate the spray cone angle, the spray tip penetration, the SMD (Sauter mean diameter) and the drop size distribution considering the experimental conditions and the fuel properties.

An investigation on effect of geometrical parameters on spray cone angle and droplet size distribution of a two-fluid atomizer

A visual study is conducted to determine the effect of geometrical parameters of a two-fluid atomizer on its spray cone angle. The liquid (water) jets exit from six peripheral inclined orifices and are introduced to a high speed gas (air) stream in the gravitational direction. Using a high speed imaging system, the spray cone angle has been determined in constant operational conditions, i.e., Reynolds and Weber numbers for different nozzle geometries. Also, the droplet sizes (Sauter mean diameter) and their distributions have been determined using Malvern Master Sizer x. The investigated geometrical parameters are the liquid jet diameter, liquid port angle and the length of the gas-liquid mixing chamber. The results show that among these parameters, the liquid jet diameter has a significant effect on spray cone angle. In addition, an empirical correlation has been obtained to predict the spray cone angle of the present two-fluid atomizer in terms of nozzle geometries.

Accurate analysis of multicomponent fuel spray evaporation in turbulent flow

The aim of this paper is to perform an accurate analysis of the evaporation of single component and binary mixture fuels sprays in a hot weakly turbulent pipe flow by means of experimental measurement and numerical simulation. This gives a deeper insight into the relationship between fuel composition and spray evaporation. The turbulence intensity in the test section is equal to 10%, and the integral length scale is three orders of magnitude larger than the droplet size while the turbulence microscale (Kolmogorov scales) is of same order as the droplet diameter. The spray produced by means of a calibrated droplet generator was injected in a gas flow electrically preheated. N-nonane, isopropanol, and their mixtures were used in the tests. The generalized scattering imaging technique was applied to simultaneously determine size, velocity, and spatial location of the droplets carried by the turbulent flow in the quartz tube. The spray evaporation was computed using a Lagrangian particle solver coupled to a gas-phase solver. Computations of spray mean diameter and droplet size distributions at different locations along the pipe compare very favorably with the measurement results. This combined research tool enabled further investigation concerning the influencing parameters upon the evaporation process such as the turbulence, droplet internal mixing, and liquid-phase thermophysical properties.

Numerical study of global rainbow technique: sensitivity to non-sphericity of droplets

The measurement of droplet temperature and size distribution in sprays is a difficult task. To reach this aim, the global rainbow technique (GRT) has been developed on the assumption that the synthetic rainbow created by a large number of droplets is insensitive to the non-sphericity of droplets if the droplets’ orientations were sufficiently random. In order to test this assumption, numerical as well as experimental analyses of GRT are carried out by our team. As a companion to the work done in experiments, the objective of this work is to quantify the sensitivity of the GRT to the non-sphericity of droplets from a numerical aspect. Light scattering properties around the rainbow angle are investigated by using the Null-field method within a T-matrix formulation, both for a single spheroid in an arbitrary orientation and for an ensemble of spheroids in random orientations illuminated by a plane wave. Refractive index and size distribution of droplets are extracted from simulated global rainbow signals so as to quantify the sensitivity of the GRT to the non-sphericity. Exemplifying results are compiled and presented. Additionally, comparisons between the Null-field method and the generalized Lorenz-Mie theory for spheroids are also provided in this paper.

Modeling the Spray Atomization of Emulsion Embedded Agricultural Solutions

Abstract A typical agricultural chemical spray process involves atomizing a liquid stream of diluted pesticide solution through hydraulic spray nozzles that inherently produce a wide spectrum of spray droplet sizes. Finer droplets have higher potential for off-target movement or drift, which is of concern due to its potential impact on neighboring crops and livestock, sensitive ecological resources, and human health. Research by the Spray Drift Task Force and others has demonstrated that, although spray nozzle selection and application parameters are the key factors to produce the desired droplet size spectrum, the physical properties of the spray solution have significant effects on the droplet size distribution for various kinds of nozzles. One of these properties of many spray fluids is the inclusion of an oil phase in the form of an emulsion. The effect of oil-in-water emulsions on the spray droplet size distribution has been demonstrated by previous work. However, the mechanisms of this effect are largely unknown. In this study, a method to model this effect was proposed. A characteristic dimensionless number for connecting the bulk spray properties and the microscopic emulsion droplet properties was defined as the ratio between the emulsion recovery time and the spray atomization time. This study will help in the design of agricultural spray nozzles and the optimization of anti-drift spray additives.

Atomization and combustion of canola methyl ester biofuel spray

The spray atomization and combustion characteristics of canola methyl ester (CME) biofuel are compared to those of petroleum based No. 2 diesel fuel in this paper. The spray flame was contained in an optically accessible combustor which was operated at atmospheric pressure with a co-flow of heated air. Fuel was delivered through a swirl-type air-blast atomizer with an injector orifice diameter of 300 μm. A two-component phase Doppler particle analyzer was used to measure the spray droplet size, axial velocity, and radial velocity distributions. Radial and axial distributions of NO, CO, CO 2 and O 2 concentrations were also obtained. Axial and radial distributions of flame temperature were recorded with a Pt–Pt/13%Rh (type R) thermocouple. The volumetric flow rates of fuel, atomization air and co-flow air were kept constant for both fuels. The droplet Sauter mean diameter (SMD) at the nozzle exit for CME biofuel spray was smaller than that of the No. 2 diesel fuel spray, implying faster vaporization rates for the former. The flame temperature decreased more rapidly for the CME biofuel spray flame than for the No. 2 diesel fuel spray flame in both axial and radial directions. CME biofuel spray flames produced lower in-flame NO and CO peak concentrations than No. 2 diesel fuel spray flames.

An operational calculus framework to characterize droplet size populations from turbulent breakup by a small number of parameters

A systematic operational calculus framework that characterizes droplet/bubble size distributions resulting from turbulent breakup of an immiscible fluid into a carrier one is presented. The proposed formulation is derived from dynamical arguments; a finite-difference formulation of the integro-differential continuous coagulation and fragmentation equation is shown to exhibit the same structure as a discrete sequence of Mellin convolutions between the probability distribution of the evolving dispersed phase and a generic kernel. This kernel may have its physical correspondence with the probability distribution resulting from a single breakup event, e.g. a liquid ligament breakup in a ligament-mediated spray formation. The number of convolution steps in the sequence can be reduced to a single parameter. As an illustration, this procedure is applied to the exponential and the gamma distributions, obtaining as a result the Fréchet distribution earlier used by Rosin and Rammler (1934 Kolloid-Zeitschrift 67 16–26), and by Nukiyama and Tanasawa (1939 Trans. Soc. Mech. Eng. Japan 5 62–7). Thus, the framework introduced in this work provides a physical foundation for the success of the Fréchet distribution in accurately fitting experimentally measured droplet size distributions in sprays and emulsions.

Development and application of the drop number size moment modelling to spray combustion simulations

This work presents the development and implementation of spray combustion modelling based on the spray size distribution moments. In this spray model, the droplet size distribution of spray is characterised by the first four moments related to number, radius, surface-area and volume of droplets, respectively. The governing equations for gas phase and liquid phase employed are solved by the finite volume method based on an Eulerian framework. These constructed equations and source terms are derived based on the moment-average quantities which are the key concept for this work. The sub-model employed for ignition and combustion is the coupling reaction rate between Arrhenius model and Eddy Break-Up model (EBU) via a reaction progress variable. The results obtained from simulation are compared with the experimental and simulation data in the literature in order to assess the accuracy of present model. Comparing with the experimental results, present approach is capable to provide a qualitatively reasonable prediction for auto-ignition. In addition, the flame area developed during the combustion progress corresponds with the experimental data.

PRESSURE—SWIRL ATOMIZATION OFWATER-IN-OIL EMULSIONS

The pressure−swirl atomization of surfactant stabilized and natural, unstable water-in-oil emulsion fuel injected into an ambient environment is investigated experimentally. Fuel flow conditions are typical of large-scale gas turbine applications. A specialized setup generates controlled emulsions stabilized by addition of surfactants. The emulsion generation process allows control over the discrete phase (water) droplet size distribution within the emulsions. The spray droplet sizes are measured using laser diffraction and the spray pattern is evaluated using imaging and mechanical patternation. A statistically designed experimental test matrix was executed and the results were subjected to the analysis of variance. We find that emulsification can reduce or increase the average droplet size in the spray depending upon the added amount of water fraction. The atomization process itself can change the size distribution of the discrete phase depending upon the initial sizes present and the injector pressure differential. The fractions of oil and water phases were observed to vary with radial spray angle for the stabilized cases considered. For the conditions studied, the stabilized emulsions performed very similarly to natural unstable emulsions as long as the unstable emulsion was produced shortly prior to atomization (i.e., <1 sec) with sufficient shear as to result in a fine discrete phase droplet size. Overall, the results provide insight into how the emulsion properties influence their atomization. Findings show that injection pressure and emulsion discrete water fraction affect the spray droplet size distribution most substantially, while the fine emulsion water droplet distributions play a less significant role. The composition of the emulsion spray appears to vary spatially and temporally when emulsions are coarse. Stabilized and naturally unstable emulsions demonstrate similar breakup behavior during atomization.

SPRAY STRUCTURE FROM DOUBLE FUEL INJECTION IN MULTIHOLE INJECTORS FOR GASOLINE DIRECT-INJECTION ENGINES

The spray characteristics of double fuel injection in multihole injectors for direct-injection gasoline engines have been evaluated in a constant-volume chamber using iso-octane as fuel. Measurements of droplets' mean and rms velocity and their diameters were obtained using a 2D phase Doppler anemometer (PDA) at injection pressures up to 120 bar, atmospheric chamber pressures, and ambient temperatures up to 115° C Complementary spray visualization made use of a pulsed light, a CCD camera, and a high-speed video camera synchronized with the injection process. Spray images with double injection revealed that there are delay times in needle opening and closing on the order of 0.6 ms and 0.3 ms, respectively, and that dwell times less than 0.5 ms introduced a prespray prior to the second injection event, while for dwell times more than 0.5 ms, the images showed that the overall jet-spray structure in the cases of the first and second injection event remained the same as those of single injection. PDA results near the injector exit quantified the relatively strong prespray at 0.3 ms dwell time and revealed that its strength reduced with increasing dwell time so that at 1 ms there was no sign of the prespray. The effect of ambient chamber temperature on the spray velocity for double injection was found to be significant at temperatures above 90° C; results at higher temperatures, 115° C, showed a consistent reduction in the mean droplets' velocity up to 10% within the core of the first and second sprays, with a corresponding increase in the rms velocity fluctuations due to the increased vaporization rates and the droplets' momentum loss. The droplet size distribution of the double-injection spray was, in general, similar to that of single injection except during the dwell time when the droplets arithmetic mean diameter with prespray was found to be smaller than those without the prespray because of the different type of droplets and the much higher data rates in the case of the prespray.

The spray characteristics of a pressure-swirl injector with various exit plane tilts

The gasoline spray characteristics of a pressure-swirl injector were investigated with various exit plane tilts. The analysis focused on the correlation between tilt angle and flow angle. Mie-scattering technique and phase Doppler anemometry were employed to analyze the macroscopic spray development and droplet size distribution of the spray. An analytical method for mass flux estimation was applied to understand the velocity distribution at the nozzle exit. The results showed that the spray shape and velocity distribution of the spray were more asymmetrical at high tilt angles. In particular, an opened hollow cone spray was formed when the tilt angle is greater than the complementary flow angle. The pressure drop inside the spray, one of the crucial factors for the swirl spray collapse at various surrounding conditions, was attenuated in this opened hollow cone spray since the pressure inside the spray was assimilated to the surrounding air pressure. The spray collapse at high fuel temperature and back pressure conditions did not appear when the tilt angle is larger than the complementary flow angle due to the reduced pressure drop inside the spray. However, tilt angle should be optimized to fulfill the requirements of spray robustness and avoid the locally rich area. The droplet size of 70° tilted nozzle spray shows a value similar to that of the original swirl spray in the plane that includes nozzle axis and the major axis of exit surface ellipse (Major axis plane) while it shows an increased value in the plane that includes nozzle axis and the minor axis of exit surface ellipse (Minor axis plane).

Numerical Simulation of Droplet Breakup and Collision in the Solution Precursor Plasma Spraying

Finely structured ceramic coatings can be obtained by solution precursor plasma spraying. The final structure of the coating highly depends on the droplet size and velocity distribution at the injection, the evolution of the spray in the jet, and droplet breakup and collision within the spray. This article describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in the process. O’Rourke’s droplet collision model is used to take into account the influence of droplet collision. The influence of droplet breakup is also considered by implementing TAB droplet breakup models into the plasma jet model. The effects of droplet collisions and breakup on the droplet size, velocity, and temperature distribution of the solution spray are investigated. The results indicate that droplet breakup and collision play an important role in determining the final particle size and velocity distributions on the substrate.

Study on the Design Criteria of the Coaxial Injector for a LOX/Hydrogen Rocket Engine —Evaluation of Design for Injectors of the LE-5 Engine and Its Derivatives—

Based on the author’s recent study on detailed modeling of the atomization characteristics of a liquid/gas phase coaxial injector, an improved computation model for predicting the combustion performance of a LOX/hydrogen engine is proposed. The features of this model are that it allows calculation of the local rate of atomization of the LOX jet and deals with droplet size distribution of the LOX spray. Additionally the model applies the burning rate constant of LOX/hydrogen combustion derived by the author’s former experiment. Using this model, evaluations of the design criteria for the LE-5 engine, which was equipped on the H-2 launcher, and its derivative engines, which have been used with the H-2A launcher, were conducted. Furthermore, optimization of the derivative engine’s injector design to improve combustion stability and combustion performance was discussed.

Investigation and Comparison of Performance of Effervescent and Standard Pneumatic Atomizer Intended for Soluble Aqueous Coating

Effervescent atomizers belong to the group of internal mixing atomizers. The effervescent approach might be a potential alternative to traditional atomization techniques, e.g., for applications where low atomization air consumption is advantageous In this paper, performance of one proposed design of the effervescent atomizer is investigated and compared to that of a standard pneumatic atomizer. The purpose of the comparison is to evaluate the actual potential of the specific effervescent atomizer in pharmaceutical relevant aqueous coating applications. Aqueous solutions of Hypromellose 5 as well as Povidone K-90F were characterized in terms of rheological properties and surface tension. Solutions were atomized by means of a standard Schlick pneumatic atomizer as well as a customized inside-out type effervescent atomizer. Spray droplet size distributions were recorded by a Spraytec instrument. Increased shear viscosity in the range 24–836 mPa.s had a modest effect on spray mean diameters for pneumatic sprays of the Newtonian solutions of Hypromellose 5. In contrast, mean droplet diameters increased by a factor of 3–5 in pneumatic sprays of Povidone K-90F solutions 11–175 mPa.s in viscosity, where non-Newtonian behavior was observed. Further, sprays of all solutions of Povidone K-90F have considerably larger mean droplet size. The effervescent atomizer atomized low viscosity solutions of Povidone K-90F more efficiently than Hypromellose 5 solutions of corresponding shear viscosity. However, atomization of high viscosity Povidone K-90F results in a coarser spray than that of the corresponding Hypromellose 5 solution. Viscosity, visco-elasticity, and surface tension of solutions all seem to affect atomization efficiency. The pneumatic atomizer was not sensitive to changes in airflow above 8.4 kg/h and liquid flow only had a considerable effect at suboptimal air flows. In its current design the effervescent atomizer improved efficiency throughout the investigated range of air flow of 0.18–0.84 kg/h and consistently produced smaller drops at liquid flow of 10 g/min compared to 35 g/min. In spite of the very low level of air consumption, the effervescent atomizer can produce fine sprays. Within its working range, the standard pneumatic atomizer, however, is capable of producing sprays of even smaller mean droplet size. All together this suggests the described effervescent atomizer as an alternative for applications where advantages of reduced atomization air flow outweigh the disadvantages of a less fine spray, e.g., in coating of attrition- prone substrate.

Direct measurement of entrainment during nanoparticle synthesis in spray flames

Using phase Doppler anemometry the scale of turbulence and the spray dilution by air entrainment are investigated during nanoparticle synthesis by flame spray pyrolysis (FSP). Ethanol and a solution of 0.5 M zirconium propoxide in ethanol are dispersed and combusted using an external-mixing, gas-assisted atomizer resulting in product ZrO 2 particles of about 11 nm in diameter as determined by nitrogen adsorption at a production rate of 100 g/h. Solution droplet size distributions of the solid cone spray are measured and related to standard correlations of spray atomization. Air entrainment and the radial spread of the expanding jet are determined from the gas velocities in horizontal planes across the spray cone at different heights above the nozzle. The isotropy of the turbulence is investigated using measured axial and radial velocity fluctuations. The turbulent flow is characterized by the integral time and Kolmogorov scales as well as the average shear rates acting on droplets and particles. The flow structure of these spray flames is of major importance as it determines product particle size, polydispersity, morphology, and crystallinity.

Multiple-scattering effects on infrared scattering measurements used to characterize droplet size and volume fraction distributions in diesel sprays

We quantify the maximum error due to multiple-scattering effects for an infrared scattering droplet izing technique. Errors in Sauter mean diameters (SMDs) and liquid volume fractions were estimated lased on experimentally determined polarization properties of the scattered light. Light that is multiply scattered from spherical particles becomes randomly polarized, whereas singly scattered light from a spherical particle contains no cross-polarization scattering component. Therefore measurement of the cross-polarization component (in this case parallel) of the scattering signal is a measure of the multiply scattered light. A ratio of parallel to perpendicular polarized scattered light was experimentally determined and used to calculate an error due to multiple scattering. The infrared scattering measurements and polarization measurements used to quantify the multiple-scattering errors were applied to a typical diesel spray that was injected into three different background conditions: a room ambient condition; a room-temperature, high-pressure condition; and a combusting condition. Droplet SMD, liquid volume fraction, and multiple-scattering errors were determined for a number of locations within the spray; results indicate that the combusting case is negligibly affected by multiple scattering. However, the room ambient case exhibited notable errors due to multiple scattering near the centerline of the spray, and the high-pressure case demonstrated susceptibility to multiple scattering throughout all regions investigated. It is important to note, however, that multiple-scattering errors in many cases translate into relatively small effects on the reported droplet sizes.

Droplet and Particle Dynamics during Flame Spray Synthesis of Nanoparticles

Particle formation by flame spray pyrolysis is investigated theoretically and compared with particle size distribution data of ZrO2 production at 100−300 g/h. Spray droplet size distributions as we...

Factors Influencing the Performance of Spray Delivery Systems: A Review of Recent Developments

Abstract The need for designers and users of pesticide application equipment to balance the risk of off-target contamination with high product efficacies when using the minimum dose of active ingredient has led to the requirement for more data defining the performance of such equipment. Measurements of the droplet size distribution in sprays from conventional nozzles has shown acceptable levels of consistency between data obtained with different measuring systems enabling such sprays to be classified providing that systems are calibrated with agreed reference nozzles. For sprays with air inclusions different measuring systems have been shown to give comparable and consistent results, and there is now a need to fully integrate such sprays into a revised classification scheme. Temperature of both spray liquid and the surrounding air have been shown to influence droplet size distribution measurements. It is therefore proposed that measurement protocols specify a maximum difference in temperature between the spray liquid and surrounding air of 5°C. For boom sprayers it is not possible to predict accurately the risk of drift from droplet size measurements alone because of the complex detrainment mechanisms involved. It is proposed that standardized wind tunnel procedures be defined that enable the relative risk of drift from different nozzle designs to be quantified. In the future it is likely that more of the information relating to application system performance will be delivered pre-programmed in the units control system or as part of a decision support system.

Experimental and Numerical Investigation of Mist Cooling for the Electra Hibachi

An experimental and numerical investigation has been conducted to examine the effectiveness of gas/liquid mist as a means of cooling the Electra hibachi structure. The aim is to quantify the effect of various operating and design parameters, viz. gas/liquid combination, gas velocity, liquid mass fraction, liquid atomization nozzle design (i.e. spray geometry, cone angle, and droplet size distribution), and heat flux on mist cooling effectiveness. The data are used to validate a mechanistic model which can be used to predict the hibachi foil’s response under prototypical pulsed operating conditions.A fully-instrumented experimental test facility has been designed and constructed. The facility includes three electrically-heated test sections, including a channel with prototypical Electra hibachi dimensions. Water is used as the mist liquid, with air, or helium, as the carrier gas. Three mist generating nozzles with significantly different spray characteristics are used. Values of the local heat transfer coefficient along the channel surface are measured for a wide range of operating conditions. The data indicate that mist cooling can increase the heat transfer coefficients by nearly an order of magnitude compared to forced convection using only the carrier gas. Comparison has been made between the data and predictions of a mechanistic three-dimensional computer program for transient two-phase flow in the channel coupled with heat conduction in the surrounding structure; excellent agreement has been obtained. The results indicate that gas/liquid mist can effectively cool the Electra hibachi structure within the design constraints imposed on circulating power requirements.

Droplet and Particle Dynamics during Flame Spray Synthesis of Nanoparticles

Particle formation by flame spray pyrolysis is investigated theoretically and compared with particle size distribution data of ZrO2 production at 100−300 g/h. Spray droplet size distributions as well as droplet and gas velocities are measured by phase Doppler anemometry along the axis and radius of the solid-cone-type spray. Multicomponent droplet combustion is simulated, accounting for droplet polydispersity, transport, and evaporation. The dynamics of the product particle size distributions are investigated, accounting for coagulation and sintering by population balance equations. The effect of the initial droplet size and liquid feed rate on the evolution of the product particle size and polydispersity is analyzed.